1. Field of the Invention
The present invention relates to a liquid-crystal panel for optically compensating wavefront aberration, and more particularly, to a liquid-crystal panel for optically compensating wavefront aberration produced due to a tilt of an optical axis of an optical beam relative to a recording surface of a recording medium. The present invention also relates to an optical pickup and a reproducing system equipped with a liquid-crystal panel.
2. Description of the Related Art
A generally known liquid-crystal panel has electrodes on both surfaces of a layer of a liquid crystal material for compensating wavefront aberration produced due to a tilt of an optical axis. The orientation of the liquid crystal molecules varies depending on the applied voltage. Using this characteristic, the liquid-crystal panel compensates wavefront aberration by changing the index of refraction of the liquid crystal material for the optical beam transmitted through this liquid crystal material. In fact, different voltages are applied to different portions of the liquid-crystal panel, and thus the liquid-crystal panel has various refractive indices at the different portions for the optical beam. As a result, the optical path length and phases of the optical beam are different at individual portions of the liquid-crystal panel. This varies the optical path length to the information-recording surface, thus canceling the effects of a tilt of an optical axis.
In the liquid-crystal panel described above, as the applied voltage is varied, the phase difference that can be imparted to the transmitted light beam varies. Depending on the characteristics of the liquid-crystal panel, the phase difference may vary non linearly in response to variations in the voltage. As a result, it is necessary to vary the applied voltage in a complex manner to control the imparted phase difference. The control system for the liquid-crystal panel is, therefore, complex.
Furthermore, depending on the characteristics of the liquid-crystal panel, the amount of wavefront aberration may exceed the range of phase difference applied by voltage to the light beam. Thus, it may be impossible to sufficiently compensate for the generated wavefront aberration.
Accordingly, the present invention is directed to a liquid-crystal panel, optical pickup and information reproducing system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a liquid-crystal panel that is capable of applying the necessary and sufficient phase difference to an optical beam by a simple voltage control method.
Another object of the invention is to provide an optical pickup equipped with the liquid-crystal panel described above.
A further object of the invention is to provide an information reproducing system equipped with the liquid-crystal panel described above.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes a liquid-crystal panel placed in an optical path of an optical beam from a light source to a recording surface of a recording medium, for compensating wavefront aberration of the optical beam by giving a phase difference to the optical beam, the liquid-crystal panel comprising a layer of a liquid crystal material having an index of birefringence, wherein a product of the index of birefringence and thickness of the layer of the liquid crystal material is 0.4 xcexcm or more.
In another aspect, the present invention includes a liquid-crystal panel placed in an optical path of an optical beam from a light source to a recording surface of a recording medium, for compensating wavefront aberration of the optical beam by giving a phase difference to the optical beam, the liquid-crystal panel comprising a layer of a liquid crystal material having an index of birefringence, wherein a product of the index of birefringence and a thickness of the layer of the liquid crystal material is set to at least an amount of wavefront aberration to be compensated.
In a further aspect, the present invention includes a liquid-crystal panel placed in an optical path of an optical beam from a light source to a recording surface of a recording medium, for compensating wavefront aberration of the optical beam by giving a phase difference to the optical beam in accordance with an applied voltage, the liquid-crystal panel comprising a layer of a liquid crystal material having an index of birefringence, wherein a product of the index of birefringence and a thickness of the layer of the liquid crystal material is set so that the phase difference varies substantially linearly with variations in the applied voltage within a range of the phase difference corresponding to an amount of wavefront aberration to be compensated.
In another aspect, the present invention includes an optical pickup comprising a light source for emitting an optical beam; a liquid-crystal panel placed in an optical path of the optical beam from the light source to a recording surface of a recording medium, for compensating wavefront aberration of the optical beam by giving a phase difference to the optical beam in accordance with an applied voltage, the liquid-crystal panel including a layer of a liquid crystal material having an index of birefringence, a product of the index of birefringence and thickness of the layer of the liquid crystal material being 0.4 xcexcm or more; an objective lens for focusing the optical beam transmitted through the liquid-crystal panel onto the recording medium; and a light-receiving means for receiving the optical beam reflected from the recording medium and for producing an output signal representing the received beam.
An information reproducing system comprising a light source for emitting an optical beam; a liquid-crystal panel placed in an optical path of the optical beam from the light source to a recording surface of a recording medium, for compensating wavefront aberration of the optical beam by giving a phase difference to the optical beam, the liquid-crystal panel including a layer of a liquid crystal material having an index of birefringence, a product of the index of birefringence and thickness of the layer of the liquid crystal material being 0.4 xcexcm or more; a voltage application means for applying a voltage across the liquid-crystal panel to give the phase difference to the optical beam; an objective lens for focusing the optical beam transmitted through the liquid-crystal panel onto the recording medium; a light-receiving means for receiving the optical beam reflected from the recording medium and for producing an output signal representing the received beam; and a reproducing means for reproducing information from the output signal from the light-receiving means.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only are not restrictive of the invention, as claimed.